'cabal repl': require Cabal version to be >= 1.18.

Fixes #1478.
Merge pull request #1480 from 23Skidoo/issue-1478

'cabal repl': require Cabal version to be >= 1.18.
Wording.
'cabal-sdist': include extra-doc-files.

Fixes #1477.
Merge remote-tracking branch 'upstream/master'
fixed typo..